1. Field of the Invention
The invention is based on a thermal turbomachine having a rotor, a stator, an abradable layer located on the stator and at least one row of rotor blades which are arranged opposite the stator around the circumference of the rotor.
2. Discussion of Background
The guide veins and rotor blades of gas turbines or compressors are exposed to strong loads. To keep the leakage losses from the thermal turbomachine at a low level, the rotor blade of the turbomachine is matched to the stator in such a manner that a stripping action occurs. A honeycomb structure is arranged at the stator of the gas turbine or compressor, opposite the rotor blade. A compressor having a honeycomb structure of this type is known, for example, from U.S. Pat. No. 5,520,508. The rotor blades of the compressor work their way into this structure, so that a minimal sealing gap is established between the rotor blades and the honeycomb structure. The honeycomb structure consists of a heat-resistant metal alloy. It is composed of a plurality of strips of sheet metal which are bent so as to match the subsequent shape.
The blade tips which abrade into an abradable structure of this type are generally provided with an abrasive layer in order to prevent or at least minimize the wear to or shortening of the rotor blade. U.S. Pat. No. 5,704,759, U.S. Pat. No. 4,589,823 and U.S. Pat. No. 5,603,603 have disclosed, by way of example, turbine blades which are equipped with abrasive materials at the blade tips.
Furthermore, U.S. Pat. No. B1 6,194,086 has disclosed an abrasive protective layer in which cubic boron nitride embedded in a matrix is applied to a turbine blade by means of a plasma spraying process.
It has been found that abrasive layers with very good cutting properties have only a very short service life of as little as just a few hours. However, the base material of the blades is usually somewhat unsuitable to being incorporated without protection in the coating at the stator, since it can melt during the abrasion process and can then be deposited or rubbed onto the stator side. When deposition of the blade material of this nature has occurred, the abrasive system is disrupted and the blades are shortened as the abrasive process continues. In the case of industrial gas turbines, approx. 80% of the abrasion depth which results in the abradable layer of the stator as a result of the rotor blades is reached within the first hours after recommissioning as a result of the abrasion procedure. After the abrasion procedure has been completed, stripping of the veins on the stator is rare, and if it does occur it only involves low penetration depths.
For this reason, it is known from U.S. Pat. No. 4,671,735 and/or DE-A1 34 01 742 for individual blades which, at their end region assigned the casing, are configured in the form of covering strips and the covering-strip-like blade end region of which bears a radially outer wear-resistant layer, to be arranged distributed over the circumference of the rotor. The layer is selected from the group of hard materials.